The invention relates to cleaning processes and systems. In particular, the invention relates to fluoride ion cleaning.
Aeronautical and power generation turbine components, such as blades, shrouds, and vanes, are often formed from superalloy materials, including but not limited to, nickel-, cobalt-, and iron-nickel-based superalloy materials. During service, turbine components are exposed to high pressure and high temperature environments and may form complex, chemically stable, thermal oxides. These oxides comprise, but are not limited to, oxides of aluminum, titanium, chromium, and combinations thereof.
Turbines are periodically overhauled in order to prolong life or enhance performance. During these overhauls, the turbine components may be subjected to various repair operations, including welding or brazing. The presence of chemically stable thermal oxides reduces the ability of a superalloy to be welded or brazed. Therefore, removal of these oxides by cleaning the turbine components prior to repair is important for successful turbine overhaul.
When only superficial repairs are required, grit-blasting or grinding can effectively remove surface oxides, although, these cleaning operations can result in inadvertent and undesirable loss of the base alloy, compromising turbine efficiency and reliability. To avoid outright excavation of the affected areas, repair of hard-to-reach surfaces, including internal passages and highly concave sections, such as cooling holes, cracks, and slots, generally requires a cleaning process that minimally degrades or damages the base alloy.
Batch thermo-chemical cleaning processes have been proposed for cleaning turbine components. Batch thermo-chemical cleaning processes attempt to remove oxides from crevices and hard-to-reach surfaces, while leaving the base alloy intact. The chemically stable oxides are generally resistant to conventional cleaning processes, such as, but not limited to, vacuum- and hydrogen-reduction or acid- and caustic-etching.
Several high-temperature, reactive-atmosphere batch cleaning processes have been proposed to affect cleaning of chemically stable oxides from turbine components. These processes generally rely on the high reactivity of fluoride ions. Processes that use fluoride ions for cleaning are collectively known as xe2x80x9cfluoride ion cleaningxe2x80x9d (FIC) processes.
Variants of the FIC process include a xe2x80x9cmixed-gas process,xe2x80x9d that employs a hydrofluoric (HF)/hydrogen (H2) gas mixture; a xe2x80x9cchromium fluoride decomposition process,xe2x80x9d that employs solid chromium fluoride and hydrogen gas for cleaning; and a xe2x80x9cfluorocarbon decomposition process,xe2x80x9d that employs polytetrafluoroethylene (PTFE) and hydrogen gas for cleaning. FIC processes are conducted at elevated temperatures, where solid (s) metal oxide (MO) is converted to vapor-phase (v) metal fluoride (MF) following a reaction having the general form:
2HF(v)+MO(s)xe2x86x92H2O(v)+MF(v)xe2x80x83xe2x80x83(1)
Differences between the various FIC processes include the fluoride ion source, reaction temperature, and reaction control mechanisms, and the composition of reaction byproducts. These differences, in turn, define a cleaning capability of each cleaning process. Both the fluorocarbon decomposition and chromium fluoride decomposition processes rely on finite sources of fluoride (PTFE or chromium fluoride, respectively). Prolonged process cycles can exhaust the fluoride source, causing the cleaning reaction to stop prematurely. The conventional mixed-gas FIC process uses an external, gaseous HF source and provides continuous control of fluoride activity through adjustment of the HFxe2x80x94H2 ratio.
Accordingly, a need for an enhanced FIC process for cleaning articles exists.
A cleaning method and system are provided for in the invention. The method comprises placing the article in a chamber, subjecting the article to a gaseous atmosphere consisting essentially of carbon, hydrogen, and fluorine; and heating the article to a temperature in a range greater than about 1500xc2x0 F. (815xc2x0 C.) to about 2000xc2x0 F. (1100xc2x0 C.) to affect cleaning of the article.
The invention also sets forth a system for cleaning articles. The system comprises means for defining a chamber; means for subjecting the article to a gaseous atmosphere, the gaseous atmosphere consisting essentially of carbon, hydrogen, and fluorine; and means for subjecting the article to the gaseous atmosphere at a temperature in a range greater than about 1500xc2x0 F. (815xc2x0 C.) to about 2000xc2x0 F. (1100xc2x0 C.) to clean the article.
These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, where like parts are designated by like reference characters throughout the drawings, disclose embodiments of the invention.